1. FIELD OF THE INVENTION
The present invention relates to an improved process for cross-linking and extrusion molding a molding material comprising a cross-linkable thermoplastic polymer admixed with a cross-linking agent while causing the molding material to flow under a high pressure.
2. DESCRIPTION OF THE PRIOR ART
It is known to incorporate in various thermoplastic polymers, such as high density and low density polyethylene, a free radical cross-linking agent, such as an organic peroxide. The organic peroxide is uniformly distributed in the polymer under conditions such that a substantial cross-linking of the polymer does not occur and then the polymer is molded. During or after molding of the polymer, the temperature of the molding material is raised to a level such that the cross-linking agent decomposes to form free radicals which cause the cross-linking reaction to occur.
The thermoplastic polymers that are known to be useful in this process include polyolefins, such as polyethylene and polypropylene; polyvinyl compounds, such as polyvinyl chloride and polyvinyl chloroacetate; polyacrylonitrile; polyesters; polyepoxides; as well as mixtures, copolymers and graft-copolymers of constituents of such polymers, such as copolymers of ethylene and vinyl monomers copolymerizable with ethylene.
The cross-linking agents that are known to be useful in this process are compounds capable of yielding free radicals when heated to a certain temperature, preferably organic peroxides, hydroperoxides, peracids and peresters. Presently preferred cross-linking agents include di-t-butyl peroxide, 2,5-dimethyl-2,5-di(t-butyl peroxy)-3-hexyne and dicumyl peroxide.
In the prior art, for the purpose of extrusion molding of cross-linkable thermoplastic polymers, it has been common to employ a method comprising feeding through a pre-forming die a molding material which has been melted by applying a temperature and a pressure insufficient to cause a cross-linking reaction in a screw-type extruder, then heating the pre-formed molten material up to a temperature higher than that necessary for starting the cross-linking reaction by means of a heating medium within a cross-linking pipe connected with the preforming die, promoting the cross-linking reaction by applying a pressure of about 20 Kg/cm.sup.2 in order to restrain the growth of voids in the molding and subsequently introducing the thus-reacted molded material into a cooling medium also under an elevated pressure thereby to cool and solidify it. The employment of this conventional method is most popular in the extrusion-molding of cross-linkable thermoplastic polymers.
As the heating medium, steam has hitherto been popular, but nowadays CO gas, N.sub.2 gas and SF.sub.6 gas are also employed. As the heating means, the use of an electric heater capable of imparting heat energy from the outside has been most prevalent. However, inasmuch as the thermal conductivity of thermoplastic polymers is so low that much time is required to raise the temperature of the inner part of moldings of such polymers uniformly by this conventional heating means, large expenses have been incurred in providing the equipment associated with the cross-linking pipe. In addition, from the view point of the quality of product, inasmuch as the molding is performed under a relatively low pressure, the void content of the resulting molded product is not always satisfactory, and especially in the case of producing high-tension cables, this would cause a serious problem.
With a view to reducing the time required for cross-linking, such processes as applying infrared heating or ultrasonic-wave heating within the cross-linking zone have been proposed. However, all these processes are intended to heat the materials after they exit from the extruding machine, and although the time required for cross-linking can be reduced thereby compared with the conventional processes, no improvement on the void content is achieved thereby, and therefore, these processes merely represent improved methods for heating the molding in the cross-linking pipe.